This invention relates to the manufacture of photoflash lamps and, more particularly, to flashlamps containing filament-supporting lead-in wires.
Photoflash lamps generate an actinic light output by the burning of an energetic fuel, such as finely shredded zirconium, hafnium or aluminum metal foil, in a combination supporting atmosphere, such as oxygen. In some of the tubular electrically ignitable photoflash lamps presently manufactured, the ignition means comprises a pair of lead-in wires sealed through one end of the tubular glass envelope. A tungsten filament is mounted across the inner ends of the two lead-in wires with the ends of the wires at their junctions with the filament being coated with a primer material, such as a powdered zirconium mixture. Typically, the envelope is comprised of G-1 type soft glass having a coefficient of thermal expansion within the range of 85 to 95 .times. 10.sup.-.sup.7 per .degree.C between 20.degree.C and 300.degree.C, and the lead-in wires are formed of a metal having a similar coefficient of thermal expansion so as to provide a match seal.
When battery power is applied to the external projecting portions of the two lead-in wires, the filament glows to incandescence, causing the primer material to ignite, which in turn ignites the finely shredded metallic combustible in the lamp envelope and, thus, flashes the lamp. During lamp flashing, the glass envelope is subjected to severe thermal and mechanical shock due to hot globules of metal oxide impinging on the walls of the lamp. As a result, cracks and crazes occur in the glass and, at higher internal pressures, containment becomes unlikely. In order to reinforce the glass envelope and improve its containment capability, it has been common practice to apply a protective lacquer coating on the lamp envelope by means of a dip process. To build up the desired coating thickness, the glass is generally dipped a number of times into a lacquer solution containing a solvent and a selected resin, typically cellulose acetate. After each dip, the lamp is dried to evaporate the solvent and leave the desired coating of cellulose acetate, or whatever other plastic resin is employed.
In the continuing effort to improve light output, higher performance flashlamps have been developed which contain higher combustible fill weights per unit of internal envelope volume along with higher fill gas pressures. In addition, the combustible material may be one of the more volatile types, such as hafnium. Such lamps, upon flashing, appear to subject the glass envelopes to more intense thermal shock effects, and thus require stronger containment vessels. One approach to this problem has been to employ a hard glass envelope, such as the borosilicate glass envelope described in U.S. Pat. No. 3,506,385, along with a protective dip coating. More specifically, this patent describes an electrically ignitable lamp having in-leads of a metal alloy such as Kovar secured by an internal expansion match seal in a lead-free glass envelope having a coefficient of thermal expansion in the range of 40 to 50 .times. 10.sup.-.sup.7 per .degree.C. Type 7052 glass is mentioned as typical. The patent imposes a minimum of 40 .times. 10.sup.-.sup.7 per .degree.C on the coefficient of thermal expansion of the glass to assure the necessary match seal with the Kovar in-leads. Further, it is theorized that glass in this thermal expansion range provides a more beneficial mode of fracture which results in a delay in crack time after flashing. More specifically, fracture of the glass is delayed to a time when the pressure in the lamp has been reduced to a point where containment is more readily assured.
As described in U.S. Pat. No. 3,832,124, assigned to the present assignee, it has been discovered that by using glasses having an even lower thermal expansion than that specified in the aforementioned U.S. Pat. No. 3,506,385 the flashlamp envelope can be made even more resistance to thermal shock and thereby delay crack time even further. Alternatively, the use of lower thermal expansion glass provides a lamp capable of higher thermal loadings, as the glass surface stresses are proportional to the thermal expansion of the glass. In particular, we have found that glasses having a coefficient of thermal expansion within the range of 30 to 40 .times. 10.sup.-.sup.7 per .degree.C between 0.degree.C and 300.degree.C are particularly suitable for improving the containment of flashlamp envelopes. Hereinafter, such glass will be referred to as "low-expansion glass". Of course, fused quartz has a very low coefficient of thermal expansion, in the order of 4 .times. 10.sup.-.sup.7 per .degree.C, but it is somewhat costly for this application.
In attempting to use a low-expansion glass envelope in flashlamp applications, however, a sealing problem arises as many of the metals typically used for ignition structures have a substantially higher coefficient of thermal expansion than the glass and, therefore, are not suitable for providing a match seal. In the above-referenced U.S. Pat. No. 3,832,124, which describes a percussive type flashlamp having a depending primer tube assembly for ignition, this problem is met by employing a compression seal. That is the metal primer tube is shaped so that it bears against the exterior surface of the glass envelope of lower thermal expansion, whereby the seal area of the glass is placed under compression upon cooling from the sealing process. Under a compressive strain, glass is made considerably stronger; hence, even though the metals are mismatched with respect to thermal expansion, a strong seal results. The low-expansion glasses suggested in that patent are Corning types 7740, 7760, 7250 and 7070, and the primer tube metals include 42 nickel-iron alloy and alloys of iron, nickel and cobalt, such as Kovar.
Another approach to the metal-to-glass mismatch problem is to employ a graded seal, such as described in U.S. Pat. No. 3,771,941, assigned to the present assignee. This last-mentioned patent describes a percussive flash lamp having a low-expansion glass, such as Corning type 7070 hard glass, sealed to a Kovar primer tube by means of a graded seal having an intermediate expansion glass, such as Corning type 7050, disposed between the envelope and primer tube.
In addition to the sealing difficulties encountered with low-expansion glasses, another major problem is workability. A common characteristic of the aformentioned hard glasses is a relatively narrow temperature working range. Accordingly, such glasses are more difficult to draw, and thus, relatively expensive. Further, such glasses are more difficult to work with conventional sealing methods during the lamp-making process, thereby causing increased shrinkage in the factory.